Low noise amplifiers (LNA) for multi-standard RF applications must operate in different frequency ranges, particularly with input signal dynamics that may be 90 dB or more. To avoid a rather costly approach of using several distinct amplifiers, with each amplifier having a selection switch external to the chip, it is necessary to overcome a number of problems deriving from the different operating frequencies, as well as from the extremely variable input signal dynamics.
To ensure an acceptable voltage standing wave ratio (VSWR), an amplifier requires a specific input matching adapting network. Such a network may normally be designed for a relatively narrow band, and optimized for a certain operating frequency. Different requirements apply to output matching because there are not relevant noise contributions that may originate from the output matching network. The matching is often achieved using resistive elements, and thereby with broadband characteristics.
Beside the above mentioned problem, the different gain and linearity specifications of each application dictate different design approaches for each amplifier. This often makes it impossible to not only share the same matching network, but even use the same active circuit.
When the gain and linearity requirements are linked to broad input dynamics, a typical approach is to make a variable gain amplifier. Though gain variation may address, to some extent, the linearity problems relative to the output section by attenuating the output signals, it does not resolve the linearity problems of the input section. The linearity problems of the input section are inherent to the nonlinear voltage-current characteristic of metal oxide semiconductor (MOS) transistors, as well as bipolar junction transistors (BJT).
Because the low-noise specification requirements must be met, a gain that is programmable in discrete steps, e.g., two or three steps, is usually preferred. Continuously programming over a broader dynamic range is usually applied to a variable gain amplifier (VGA) downstream of the mixer stage. An LNA circuit normally used to ensure the input matching and programmability of the gain is illustrated by the diagram of FIG. 1.
The current generator I.sub.PTAT and the current mirror Q.sub.1 -Q.sub.2 establish the operating current in Q.sub.2 to ensure stability in the gain when the temperature changes. A portion of the current signal, generated by Q.sub.2, reaches the resistive load RL via Q.sub.3. The portion of the current that flows in Q.sub.4 is dispersed in the supply rail. Resistors R.sub.A and R.sub.B have the function of reducing the noise contribution originating from the bias network, and permits a simpler application of the signal through the coupling capacitor C.
The current signal on the load resistor depends on the voltage on the control terminal VC. If VC is much lower than V.sub.B (V.sub.B &gt;V.sub.C +4V.sub.T), all the signal current of Q.sub.2 goes to Q.sub.3 and is converted to a voltage on the resistor R.sub.L producing the maximum gain. In the opposite case, if the VC voltage is greater than V.sub.B (V.sub.C &gt;V.sub.B +4V.sub.T), the entire current flows in Q.sub.4 and the gain is zero. By properly setting V.sub.C it is possible to obtain any gain value.
As mentioned above, to reduce the noise figure, the gain cannot be lowered below a minimum value. This requires an accurate control of the V.sub.C voltage. The practical aspect of a sufficiently accurate control is not easy because of the exponential voltage-current relation in the Q.sub.3 -Q.sub.4 pair.
The inductors L.sub.E and L.sub.B in the circuit of FIG. 1 perform the impedance matching for the real part and the imaginary part of the input impedance, Z.sub.in, respectively. By hypothesizing that ##EQU1##
the condition for the matching of the input impedance, Z.sub.in (jw)=R.sub.s, implies the selection of L.sub.E and L.sub.B values that satisfy the following equations: EQU .omega..sub.T.multidot.L.sub.E +r.sub.b.congruent.R.sub.s (1) EQU .omega..sup.2.multidot.(L.sub.B +L.sub.E)C.sub..pi..congruent.1 (2)
The linearity of the voltage-to-current transformation through Q.sub.2 is improved by the presence of L.sub.E, which causes an emitter degeneration. Therefore, to improve the linearity in the presence of an ample input signal, L.sub.E is incremented. However, this is a detriment for matching of the real part of Z.sub.in. An imperfect input impedance match for linearity is acceptable for signals having a sufficient magnitude.